Quantum-confinement stark effect optical modulator

ABSTRACT

A quantum confinement Stark effect (QCSE) optical modulator has a ridge stripe including a current confinement structure. The current confinement structure includes an AlInAs layer which is subjected to selective oxidation of Al content in the AlInAs layer. The current confinement structure is such that a pair of Al-oxidized regions of the AlInAs layer sandwiches therebetween a central un-oxidized region.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a quantum-confinement Starkeffect (QCSE) optical modulator, and an integrated semiconductor opticaldevice including a semiconductor laser device and a QCSE opticalmodulator integrated on a single chip.

[0003] More specifically, the present invention relates to such a QCSEoptical modulator or a semiconductor optical device having a lowerdevice resistance and an excellent frequency characteristic.

[0004] The present invention also relates to a method for fabricatingthe QCSE optical modulator and an integrated semiconductor opticaldevice including the QCSE optical modulator and a semiconductor laserdevice.

[0005] (b) Description of the Related Art

[0006] In a super-lattice structure, if an electric field is appliednormal to the layers, excitons are hardly dissociated in a moderateelectric field due to the presence of a barrier layer preventing thedissociation of the excitons. For example, if an electric field of 10⁴volts/cm is applied to a quantum well having a width of 10 nm, thequantum well is inclined by an amount corresponding to 10 meV. Thisrange of the electric field scarcely causes the excitons in the quantumwell to dissociate, and only a peak of the optical absorption spectrumis observed to shift toward a lower energy level. This phenomenon iscalled QCSE.

[0007] A QCSE optical modulator including an AlGaInAs-based quantum wellstructure and taking advantage of the QCSE is described in “Journal ofLightwave Technology”, vol. 8, No. 7, July 1990, and recited to have alower operational voltage and achieve a higher-speed modulation comparedto GaInAsP-based QCSE optical modulator

[0008]FIG. 1 shows a conventional QCSE optical modulator (may bereferred to as simply “optical modulator” hereinafter). The opticalmodulator is of a waveguide type and includes a p-i-n structure formedon an InP substrate 112, the p-i-n structure being such thatAlGaInAs/AlInAs multiple quantum well (MQW) 118 constituting anintrinsic layer is sandwiched between a p-type cladding layer 120 and ann-type cladding layer 116.

[0009] The optical modulator 110 has the n-InP substrate 112 and a layerstructure including an n-type InP (n-InP) layer 114, the n-AlInAscladding layer 116, the MQW 118, the p-AlInAs cladding layer 120, and ap-InGaAs contact layer 122, which are consecutively grown on the n-InPsubstrate 112 by a molecular beam epitaxy (MBE). A p-side electrode 124and an n-side electrode 126 are formed on the p-type contact layer 122and the bottom surface of the InP substrate 112, respectively.

[0010] The MQW 118 includes a plurality (30) of film pairs eachincluding a 86-angstrom-thick AlGaInAs quantum well layer and a50-angstrom-thick AlInAs barrier layer and formed in a cyclic order. Then-AlInAs cladding layer 116, MQW 118, p-AlInAs cladding layer 120 andp-InGaAs contact layer are configured as a ridge stripe of a higher mesastructure having a width of 4 μm and a length of 90 to 120 μm.

[0011] The p-InP layer 114 and the ridge stripe are covered by a SiO₂film except for the p-side electrode 124.

[0012] When a reverse bias voltage is applied to the conventionaloptical modulator 110 of FIG. 1, the QCSE shifts the peak of the opticalabsorption spectrum of excitons toward the longer wavelength side,thereby increasing the optical absorption effect of the opticalmodulator for the laser. This operation uses a reverse bias voltage ofthe p-i-n junction as a drive current for the change of the opticalabsorption, and thus achieves a larger change of the optical absorptionat a high speed by using a small voltage.

[0013] In the conventional optical modulator 110 as described above,there is a problem in that the higher mesa structure of the ridge stripegenerally has a rough surface formed on the ridge wall during theetching for configuring the mesa structure, the rough surface causingscattering loss of transmitted light to degrade the devicecharacteristics.

[0014] In addition, the small ridge width of the mesa structure raisesthe resistance of the p-type cladding layer, which has in general alarger resistance compared to the n-cladding layer, and raises theoverall device resistance.

[0015] Further, it is difficult to adopt a selective growth technique inthe case of integration of the conventional optical modulator with asemiconductor laser device, due to the presence of the Al content in thematerial for the MQW and the cladding layer.

[0016] For the reasons as recited above, a semiconductor optical devicehaving an optical modulator and a semiconductor laser device integratedin a single chip generally has a higher device resistance and lowerdevice characteristics.

SUMMARY OF THE INVENTION

[0017] In view of the above problems in the conventional techniques, itis an object of the present invention to provide an optical modulatorhaving a lower electric resistance and excellent device characteristics,to provide a semiconductor optical device including the opticalmodulator and a semiconductor laser device integrated in a single chip.

[0018] It is also an object of the present invention to provide such anoptical modulator and a semiconductor optical device.

[0019] The present invention provides a quantum confinement Stark effect(QCSE) optical modulator including a compound semiconductor substrate,and a layer structure formed thereon, the layer structure including anAlGaInAs-based multiple quantum well (MQW), a pair of cladding layershaving opposite conductivity types and sandwiching therebetween the MQW,and an Al-containing layer overlying the MQW or formed within one of thecladding layers having a p-type conductivity, the layer structure beingconfigured as a ridge stripe at a portion including the Al-containinglayer, the Al-containing layer having a current confinement structurewherein a pair of Al-oxidized regions of the Al-containing layersandwiches therebetween a central un-oxidized region of theAl-containing layer.

[0020] In accordance with the optical modulator of the presentinvention, the width of the cladding layers in the ridge stripe is widerand thus the device electric resistance is lower compared to theconventional optical device, due to the presence of the currentconfinement structure having a pair of Al-oxidized regions sandwichingtherebetween un-oxidized region. In addition, the presence of thecurrent confinement structure having the pair of Al-oxidized regionsprevents a rough surface of the ridge side, and thus reduces thetransmission loss of the laser. These advantages also result inimprovement of the frequency response of the optical modulator.

[0021] The Al-containing layer may be an AlInAs layer.

[0022] The present invention also provides method for fabricating aquantum confinement Stark effect (QCSE) optical modulator including thesteps of:

[0023] forming a layer structure on an InP substrate, the layerstructure including a multiple quantum well (MQW), pair of claddinglayers sandwiching therebetween the MQW and an Al-containing layeroverlying the MQW;

[0024] configuring at least a portion of the layer structure includingthe Al-containing layer to form a ridge stripe; and

[0025] selectively oxidizing Al in the Al-containing layer to form acurrent confinement structure in the ridge structure, the currentconfinement structure having a pair of Al-oxidized regions of theAl-containing layer sandwiching therebetween an un-oxidized region ofthe Al-containing layer.

[0026] In accordance with the method of the present invention, thestructure of the QCSE optical modulator optical device having a lowerelectric resistance and improved frequency characteristics can be formedwith a simple process.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings. BRIEF DESCRIPTION OF THEDRAWINGS

[0027]FIG. 1 is a perspective view of a conventional optical modulator.

[0028]FIG. 2 is a sectional view of an optical modulator according to afirst embodiment of the present invention.

[0029]FIGS. 3A to 3C are sectional views of the optical modulator ofFIG. 2 for showing consecutive steps of fabrication thereof.

[0030]FIG. 4 is top plan view of an integrated optical device accordingto a second embodiment of the present invention.

[0031]FIG. 5 is a sectional view of a semiconductor laser device formedin the optical device of FIG. 4.

[0032]FIG. 6 is a partial sectional view of the optical device of FIG.4.

[0033]FIGS. 7A to 7H are sectional views of the optical device of FIG. 4in steps of fabrication thereof.

[0034]FIG. 8 is a top plan view of an optical device according to athird embodiment of the present invention.

[0035]FIG. 9 is a sectional view of the semiconductor laser devicedisposed in the optical device of FIG. 8.

[0036]FIG. 10 is a sectional view of the optical modulator disposed inthe optical device of FIG. 8.

[0037]FIG. 11 is a partial sectional view of the optical device of FIG.8.

[0038]FIGS. 12A to 12G are sectional views of the optical device of FIG.8 showing the steps of fabrication thereof.

PREFERRED EMBODIMENTS OF THE INVENTION

[0039] Now, the present invention is more specifically described withreference to accompanying drawings.

First Embodiment

[0040] Referring to FIG. 2, an optical modulator, generally designatedby numeral 10, is an example of the first aspect of the presentinvention. The optical modulator 10 includes an n-InP substrate 12, andan AlGaInAs-based layer structure including an n-InP cladding layer 14,an AlGaInAs-based MQW 16, a GaInAsP optical confinement layer 18, ap-InP cladding layer 20, a p-AlInAs layer 22, a p-InP cladding layer 24and a p-GaInAs contact layer 26, which are consecutively grown on then-InP substrate 12.

[0041] The n-InP cladding layer 14 is 100 nm thick, and has a carrierdensity of 5×10^(17 cm) ⁻³: the MQW has a lasing wavelength (λg) of 1.52μm: the GaInAsP optical confinement layer 18 is 10 nm thick and has awavelength of 1.2 μm: the p-InP cladding layer 20 is 150 nm thick andhas a carrier density of 5×10^(17 cm) ⁻³: the p-AlInAs layer 22 is 100nm thick and has a carrier density of 1×10¹⁸ cm⁻³: the p-InP claddinglayer 24 is 2000 nm thick and has a carrier density of 1×10¹⁸ cm⁻³: andthe p-GaInAs contact layer 26 is 300 nm thick and has a carrier densityof 1×10⁹ cm⁻³.

[0042] The MQW structure includes a plurality (10) of film pairs eachincluding a 9-nm-thick GaInAs well layer and a 5-nm-thick AlInAs barrierlayer.

[0043] Among the layers in the layer structure, the p-AlInAs layer 22,p-InP cladding layer 24 and p-GaInAs contact layer 26 are configured toform a 10-μm-wide ridge stripe. On each side of the p-AlInAs layer 22, a3.5-μm-wide Al-oxidized region 28 formed by selective oxidation of Alcontent in the p-AlInAs layer 22 extends along the ridge stripe betweeneach of the edges of the un-oxidized region of the AlInAs layer 22 andeach side surface of the ridge stripe. The Al-oxidized region 28 forms acurrent confinement structure in the optical modulator.

[0044] On the layer structure including the ridge stripe, a SiN film 30is formed as an insulator/overcoat film. The SiN film 30 has on top ofthe ridge stripe an opening, through which a p-side electrode 32including Ti/Pt/Au metallic films contacts with the underlying p-GaInAscontact layer 26. An n-side electrode 34 including AuGeNi/Au metallicfilms is formed on the bottom surface of the n-InP substrate 12.

[0045] The conventional optical modulator had a 3-μm-wide ridge stripe,which limited the conductive area for the p-cladding layer having alarger inherent resistance, whereby the device resistance was as high asb 8Ω, for example.

[0046] On the other hand, a sample of the optical modulator of thepresent embodiment had a device resistance of 6.8Ω. The lower deviceresistance is due to the presence of the current confinement structureformed by the Al-oxidized regions 28, which allows the ridge stripe tohave a larger width as large as 10 μm.

[0047] The conventional optical modulator without such a currentconfinement structure exhibited a characteristic frequency of 20 GHz,whereas the sample of the optical modulator of the present embodimenthaving the current confinement structure had a characteristic frequencyof 25 GHz, thereby showing a significant improvement in the frequencycharacteristics.

Fabrication of the Optical Modulator

[0048] Fabrication of the optical modulator of the embodiment will bedescribed below with reference to FIGS. 3A to 3C. In FIG. 3A, on ann-InP substrate 12, a 100-nm-thick n-InP cladding layer 14 having acarrier density of 5×10^(17 cm) ⁻³, an AlGaInAs-based MQW 16 having alasikng wavelength of 1.52 μm, a 10-nm-thick GaInAsP optical confinementlayer 18 having a wavelength of 1.2 μm, a 150-nm-thick p-InP claddinglayer 20 having a carrier density of 5×10¹⁷ cm⁻³, and a 100-nm-thickp-AlInAs layer 22 are consecutively grown by epitaxial processes.

[0049] Subsequently, a 2000-nm-thick p-InP cladding layer 24 having acarrier density of 1×10¹⁸ cm⁻³, and a 300-nm-thick p-GaInAs contactlayer 26 having a carrier density of 1×10¹⁹ cm⁻³ are grown thereon byepitaxial processes.

[0050] Thereafter, as shown in FIG. 3B, the p-GaInAs contact layer 26,p-InP cladding layer 24, and p-AlInAs layer 22 are selectively etched toconfigure a 10-μm-wide ridge stripe, which exposes top of the p-InPlayer 20 at both sides of the ridge stripe.

[0051] The p-AlInAs layer 22 in the ridge stripe is selectively oxidizedby a known process and known conditions by progressively oxidizing thep-AlInAs layer 22 from the side surfaces toward the internal of theridge stripe, with the central region of the p-AlInAs layer 22 beingleft un-oxidized. Thus, a 3.5-μm-wide Al-oxidized region 28 is formed ateach side of the ridge stripe in the p-AlInAs layer 22.

[0052] Subsequently, a SiN film not shown is formed on the layerstructure including the ridge stripe, followed by selective removalthereof to form an opening for exposing the p-GaInAs contact layer 26 onthe top of the ridge stripe. A p-side electrode 32 including Ti/Pt/Aufilms are formed on the entire surface including the surface of thep-GaInAs contact layer 26, and an n-side electrode 34 includingAuGeNi/Au films is formed on the bottom surface of the n-InP substrate12, as shown in FIG. 3C. Thus, the optical modulator 10 of FIG. 2 can beachieved.

Second Embodiment

[0053] Referring to FIG. 4, a semiconductor optical device, generallydesignated by numeral 36, according to the present embodiment is anexample of a second aspect of the present invention.

[0054] The semiconductor optical device includes a semiconductor laserdevice 40 and a ridge-type optical modulator 10, which are coupledtogether by a coupling area 37. The structure of the semiconductor laserdevice 40 is shown in FIG. 5, and the structure of the coupling area isshown in FIG. 6.

[0055] The semiconductor laser device 40 and the optical modulator 10are coupled together by an optical waveguide 38 interposed therebetween.The semiconductor optical device has a butt-joint structure, wherein atleast one layer of the semiconductor laser device 40 is coupled to acorresponding layer of the optical modulator 10 with a butt-jointstructure formed in the optical waveguide 38.

[0056] The semiconductor laser device 40 is a distributed feedback laserdevice (DFB-LD) of a buried heterosructure (BH) including GaInAsP-basedmaterials. As shown in FIG. 5, the semiconductor laser device 40 has alayer structure mounted on an n-InP substrate 12 which also mountsthereon the optical modulator 10. The layer structure includes an n-InPcladding layer 44, a GaInAsP-based MQW 46, a GaInAsP layer 48 in which adiffraction grating is formed, an a p-InP cladding layer 50, which areconsecutively formed on the n-InP substrate 12.

[0057] Among the layers of the layer structure, a top portion of then-InP cladding layer 44, MQW 46, GaInAsP layer 48 and the p-InP claddinglayer 50 are configured as a ridge stripe. Both the sides of ridgestripe are buried by a combination of p-InP blocking layer 52 and n-InPblocking layer 54, which form a current confinement structure using ap-n junction.

[0058] A p-InP cladding layer 56 and a p-GaInAs contact layer 58 areconsecutively formed on the p-InP cladding layer 50 and the n-InPblocking layer 54 in the semiconductor laser 40. The p-InP layer 56 andthe p-GaInAs contact layer 58 are formed as common layers with the p-InPlayer 24 and the p-GaInAs layer 26, respectively, in the opticalmodulator 10.

[0059] A p-side electrode 60 including Ti/Pt/Au films is formed on thep-GaInAs layer 58, and an n-side electrode 62 is formed as a commonlayer with the n-side electrode 34 of the optical modulator 10 on thebottom surface of the InP substrate 12.

[0060] The n-cladding layer 44 is 100 nm thick, and has a carrierdensity of 5×10¹⁷ cm⁻³: the MQW 46 has a lasing wavelength (λg) of 1.55μm: the GaInAsP layer 48 is 10 nm thick, has a wavelength of 1.2 μm, andincludes therein a diffraction grating: and the p-InP cladding layer 50is 200 nm thick and has a carrier density of 5×10¹⁷ cm⁻³.

[0061] The p-InP burying layer 52 is 500 nm thick, and has a carrierdensity of 1×10¹⁸ cm⁻³: the n-InP layer 54 is 500 nm thick, and has acarrier density of 1×10¹⁸ cm⁻³: the p-InP cladding layer 56 is 2000 nmthick, and has a carrier density of 1×10⁻¹⁸ cm⁻³: and the p-GaInAscontact layer 58 is 300 nm thick, and has a carrier density of 1×10¹⁹cm⁻³.

[0062] In the coupling area 37 for the semiconductor laser device 40 andthe optical modulator 10, as shown in FIG. 6, the n-InP cladding layer14, AlGaInAs-based MQW 16, GaInAsP optical confinement layer 18, p-InPcladding layer 20, p-AlInAs layer 22, and p-InP cladding layer 23 in theoptical modulator 10 are coupled with the n-InP cladding layer 44,GaInAsP-based MQW 46, GaInAsP layer 48 including the diffraction gratingand the p-InP cladding layer 50, respectively, in the semiconductorlaser device 40 with butt-join structures. The p-InP cladding layer 23functions as a protective layer for the p-AlInAs layer 22.

Fabrication of Second Embodiment

[0063] Referring to FIGS. 7A to 7H, there are shown 1 5 fabricationsteps for manufacturing the optical device of the second embodiment.

[0064] In FIG. 7A, a layer structure for the GaInAsP-based based DFB-LD40 is formed over the entire surface of an n-InP substrate 12 by usingknown processes. More specifically, a 100-nm-thick n-InP cladding layer44 having a carrier density of 5×10¹⁷ cm⁻³, a GaInAsP-based MQW 46having a lasing wavelength of 1.55 μm and a 10-nm-thick GaInAsPwaveguide layer 48 are grown on the n-InP substrate 12 by using anepitaxial technique. Then, a diffraction grating is formed in theGaInAsP layer 48 by a known technique, followed by epitaxially growingthereon a 200-nm-thick p-InP cladding layer 50.

[0065] Subsequently, a SiN film is formed on the entire surfaceincluding the optical modulator area (which is herein designated also bynumeral 10) and the DFB-LD area (which is herein designated also bynumeral 40), followed by patterning thereof to form a SiN mask whichcovers the current injection region of the DFB-LD area 40.

[0066] By using the SiN mask, the p-InP cladding layer 50, GaInAsP layer48, MQW 46 and a top portion of the n-InP cladding layer 14 areconfigured by selective etching to form a ridge stripe Thereafter, aselective growth process is conducted by using the SiN mask as aselective growth mask to consecutively form a 500-nm-thick p-InPblocking layer 52 having a carrier density of 1×10¹⁸ cm⁻³ and a500-nm-thick n-InP blocking layer 54 having a carrier density of 1×10¹⁸cm⁻³, on both the sides of the ridge stripe, to bury the ridge stripeand form a current confinement structure for the DFB-LD 40.

[0067] By the above processes, the layer structure shown in FIG. 7B isformed in the DFB-LD area 40, whereas the layer structure shown in FIG.7C is formed in the optical modulator area 10 and the coupling area,FIG. 7C being taken as viewed along the extending direction of the ridgestripe.

[0068] Thereafter, the optical modulator 10 is formed in the opticalmodulator area. Specifically, the SiN mask 51 is removed and another SiNmask 55 is formed on the DFB-LD area 40, as shown in FIG. 7D. The layerstructure formed in the optical modulator area 10 is then subjected toetching to expose the InP substrate 12.

[0069] Subsequently, as shown in FIG. 7E, a 100-nm-thick n-InP claddinglayer 14 having a carrier density of 5×10¹⁷ cm⁻³, an AlGaInAs-based MQW16 having a lasting wavelength of 1.52 μm, a 10nm-thick GaInAsP opticalconfinement layer 18 having a wavelength of 1.2 μm, a 50-nm-thick p-InPcladding layer 20 having a carrier density of 5×10¹⁷ cm⁻³, a100-nm-thick p-AlInAs layer 22 having a carrier density of 1×10¹⁸ cm⁻³,and a 10-nm-thick p-InP cladding layer 23 having a carrier density of1×10¹⁸ cm⁻³ are consecutively grown in the optical modulator area byepitaxial processes. The p-InP cladding layer 23 functions as aprotective layer for the p-AlInAs layer 22.

[0070] In the step of growing the barrier layers in the MQW 16 and thep-AlInAs layer 22, CBr₄ gas is added in the material gas in an amount of5 to 50 mol. percents.

[0071] In the selective growth process of the typical Al-basedsemiconductor layer, the Al-based semiconductor layer generally has apoor film property due to growth of the polycrystalline substance on theSiN mask. Thus, it is preferable that CBr₄ gas be added in the materialgas as an etchant to remove the polycrystalline substance grown on theSiN mask by etching, thereby selectively growing the Al-basedsemiconductor layer having an excellent film property.

[0072] In addition, it is preferable that CBr₄ be added in the materialgas in the step of growing the barrier layers in the MQW for doping thebarrier layers with carbon to achieve a modulation-doped structure.

[0073] If the etchant as described above does not include carbon as anadditive, and includes HCl, for example, the etchant may be preferablyused for growing the well layers in the MQW for prevention of thepolycrystalline substance.

[0074] By using the above steps, the layer structure shown in FIG. 7D ismaintained in the DFB-LD area 40 whereas the layer structure shown inFIG. 7E is formed in the optical modulator area 10. In addition, thelayer structure shown in FIG. 7F is formed in the coupling area, FIG. 7Fbeing taken as viewed along the ridge stripe in the DFB-LD area 40.

[0075] Then, the p-InP cladding layer 24 and GaInAs contact layer 26(58) are formed. Specifically, the SiN mask 55 is removed from theDFB-LD area 40, followed by growth of a 2000-nm-thick p-InP claddinglayer 24 (56) having a carrier density of 1×10¹⁸ cm⁻³ and a 300-nm-thickp-GaInAs contact layer 26 (58) having a carrier density of 1×10¹⁹ cm⁻³by using an epitaxial technique.

[0076] After the above processes, the layer structure shown in FIG. 7Gis formed in the DFB-LD area 40, the layer structure shown in FIG. 3A isformed in the optical modulator area 10, and the layer structure shownin FIG. 7H is formed in the coupling area 37.

[0077] Thereafter, a current confinement structure is formed in theoptical modulator area 10. Specifically, an etching mask (not shown) isformed on the entire surface of the DFB-LD area 40 and the surface of aregion to be formed as a current injection region in the opticalmodulator area 10. An etching step is then conducted to the p-AlInAslayer 22, p-InP cladding layer 24 and p-GaInAs contact layer 26 by usingthe etching mask to form a ridge stripe, as shown in FIG. 3B, at thelocation passed by the extension extending from the current injectionregion of the DFB-LD area 40, to thereby form a 10 μm-wide ridge stripewhich exposes the p-InP cladding layer 20 in the optical modulator area10.

[0078] Then, an oxidation process is conducted from both the sides ofthe ridge stripe of the optical modulator area 10 toward the interiorthereof, thereby oxidizing the Al content in the p-AlInAs layer 22, asshown in FIG. 3B. Thus, a pair of 3.5-μm-wide Al-oxidized regions 28 areformed at both the sides of the ridge stripe, with the central region ofthe AlInAs layer 22 being remained un-oxidized.

[0079] Thereafter, a SiN film 30 is formed on the p-InP layer 20 andalong the ridge stripe of the optical modulator area 10, followed byselective etching thereof to form an opening for exposing a portion ofthe p-GaInAs layer 26.

[0080] Subsequently, p-side and n-side electrodes are formed.Specifically, the p-side electrodes 60 and 32 each including Ti/Pt/Aufilms are separately formed in the DFB-LD area 40 and the opticalmodulator area 10, respectively, as shown in FIGS. 5 and 2. The n-sideelectrode 34 including AuGeNi/Au films is formed on the bottom surfaceof the InP substrate 12 common to both the areas 40 and 10.

[0081] In an alternative of the above process, the AlGaInAs-basedoptical modulator may be formed first, then the GaInAsP-based DFB-LD maybe formed by a re-growth process. In this case, it is liable that theresultant optical device has degraded device characteristics, althoughthe disadvantage in the above embodiment that the polycrystallinesubstance is deposited on the SiN selective growth masks is removed. Thedegradation of the device characteristics are considered to result fromthe fact that there is a significant difference in the quantum wellcharacteristics between the structure formed in the vicinity of the maskedge and that formed in the area which is not affected by the mask due,as a result of the active layer being grown in the DFB-LD. Thus, it ispreferable in fabrication of the optical device of the second embodimentthat the AlGaInAs-based optical modulator be formed in the second growthstep. In this step, addition of etchant such as CBr₄ is quite effective.

[0082] In the optical device 36 fabricated by the method of the presentembodiment, since the DFB-LD 40 is formed by an ordinary fabricationprocess, the resultant DFB-LD 40 has laser characteristics similar tothose in the conventional DFB-LD. On the other hand, the opticalmodulator 10 has excellent characteristics as described before.

Third Embodiment

[0083] Referring to FIG. 8, a semiconductor optical device according tothe third embodiment includes a semiconductor laser device 70 and anoptical modulator 100 which are coupled by a coupling area 71, similarlyto the second embodiment. In this embodiment, the layers in thesemiconductor laser device 70 and the optical modulator 100 havedifferent film compositions as viewed in the thickness direction as aresult of the area-selective growth process. The layer compositions inthe semiconductor laser device 70 is similar to those in the opticalmodulator 100. FIGS. 9 and 10 show the semiconductor laser device 70 andthe optical modulator 10, respectively.

[0084] The semiconductor laser device 70 is a DF-BLD of ridge waveguidetype, and has an AlGaInAs-based layer structure including an n-InPcladding layer 74, an AlGaInAs-based MQW 76, a GaInAsP layer 78including a diffraction grating, a p-InP cladding layer 80, a p-InPcladding layer 81, a p-AlInAs layer 82, a p-InP cladding layer 84 and ap-GaInAs contact layer 86, which are consecutively grown on an n-InP sub72.

[0085] The n-InP cladding layer 74 has a thickness of 100 nm, and acarrier density of 5×10¹⁷ cm⁻³: the MQW 76 has a lasing wavelength of1.55 μm: the GaInAsP layer 78 has a thickness of 8 nm and a wavelengthof 1.2 μm: the p-InP cladding layers 80 and 81 each has a thickness of100 nm and a carrier density of 5×10¹⁷ cm⁻³: the p-AlInAs layer 82 has athickness of 100 nm and a carrier density of 1×10¹⁸ cm⁻³: the p-InPcladding layer 84 has a thickness of 2000 nm and a carrier density of1×10¹⁸ cm⁻³: and the p-GaInAs contact layer 86 has a thickness of 300 nmand a carrier density of 1×10¹⁹ cm⁻³.

[0086] The layer structure formed on the n-InP substrate 72 andincluding n-InP cladding layer 74, MQW 76, GaInAsP layer 78 includingthe diffraction grating, p-InP cladding layers 80 and 81, p-AlInAs layer82, p-InP cladding layer 84 and p-GaInAs contact layer 86 is configuredas a 10-μm-wide ridge stripe.

[0087] The p-AlInAs layer 82 has a pair of Al-oxidized regions 88 formedby selectively oxidizing the Al content therein and sandwichingtherebetween a central un-oxidized region. Each of the Al-oxidizedregions 88 has a width of 3.5 μm.

[0088] On the layer structure including the ridge stripe, a SiN film 90is formed having an opening on top of the ridge stripe. A p-sideelectrode 92 including Ti/Pt/Au films is in contact with the underlyingp-GaInAs layer 86 through the opening, and an n-side electrode 94including AuGeNi/Au films is formed on the bottom surface of the n-InPsubstrate 72

[0089] The optical modulator 100 has an AlGaInAs-based layer structureformed on the common n-InP substrate 72 and including an n-InP claddinglayer 74, an AlGaInAs-based MQW 76, a GaInAsP optical confinement layer78, p-InP cladding layers 80 and 81, a p-AlInAs layer 82, a p-InPcladding layer 84 and a p-GaInAs contact layer 86, each of which extendsfrom a corresponding one of the layers in the layer structure of thesemiconductor laser device 70 at the optical coupling area.

[0090] In the layer structure, each of the n-InP cladding layer 74, MQW76, GaInAsP layer 78 and p-InP cladding layer 80 in the opticalmodulator 100 is formed as a common layer with the corresponding one ofthe layers in the layer structure in the semiconductor laser device 70,and yet has a smaller thickness compared to the corresponding layer. Forexample, the n-InP cladding layer 74, GaInAsP layer 78, p-InP claddinglayer 80 have thicknesses of 80 nm, 6 nm and 80 nm, respectively, andthe GaInAsP layer 78 has no diffraction grating. In addition, the MQW iscontrolled to have a lasing wavelength of 1.52 μm.

[0091] Each of the p-InP cladding layer 81, p-AlInAs layer 82, p-InPcladding layer 84 and p-GaInAs contact layer 86 has a film thickness anda carrier density equal to those of the corresponding layer in thesemiconductor laser device 70.

[0092] The optical modulator 100 has a layer structure formed on then-InP substrate 72 and configured as a 10-μm-wide ridge stripe similarlyto the semiconductor laser device 70 and extending from the ridge stripeof the semiconductor laser device 70.

[0093] The p-AlInAs layer 82 includes a pair of 3.5-μm-wide Al-oxidizedregions 88 located at both sides of the ridge stripe and sandwichingtherebetween a central un-oxidized region.

[0094] On the layer structure including the ridge stripe except for thetop thereof, a SiN film 90 is formed as an insulation/protection film. Ap-side electrode including Ti/Pt/Au films is in contact with theunderlying p-GaInAs contact layer 86 through the opening of the SiN film90, and an n-side electrode 94 is formed on the bottom surface of then-InP substrate 72.

[0095] In the semiconductor optical device 66 of the present embodiment,as shown in FIG. 11, each of the layers in the optical modulator 100extends from the corresponding layer in the semiconductor laser device70, with a difference residing in the film thickness between thecorresponding films.

Fabrication of the Third Embodiment

[0096] Referring to FIGS. 12A to 12G, fabrication process for theoptical device of the present embodiment will be described hereinafter.

[0097] In FIG. 12A, a SiN area-selective growth mask 73 having aspecified mask pattern is formed on the semiconductor laser area of then-InP substrate 72. The mask pattern includes a pair of 30-μm-widestripes opposing each other with a gap “G” therebetween.

[0098] Then, a area-selective epitaxial process is conducted on the InPsubstrate 72 by using a MOCVD process to form an n-InP cladding layer74, an AlGaInAs-based MQW 76, a GaInAsP layer 78 having a wavelength of1.2 μm and a p-InP cladding layer 80 having a carrier density of 5×10¹⁷cm⁻³.

[0099] In the epitaxial process, the layer structure in the opticalmodulator area 100 which has no mask pattern therein are such that then-InP cladding layer 74, GaInAsP layer 78 and p-InP cladding layer 80have film thicknesses of 40 nm, 6 nm and 8 nm, respectively.

[0100] In the semiconductor laser area 70, due to the function of thearea-selective growth mask pattern 73, the n-InP cladding layer 74,GaInAsP layer 78 and p-InP cladding layer 80 have film thicknesses of 50nm, 8 nm and 10 nm, respectively.

[0101] The MQW 76 has a wavelength of 1.52 μm in the optical modulatorarea 100 and 1.55 μm in the semiconductor laser area 70. CBr₄ gas may bepreferably added to the material gas as an etchant during growth of theAl-based compound semiconductor layers, i.e., barrier layers for the MQW76.

[0102] Subsequently, the area-selective growth mask 73 is removed,followed by forming diffraction grating in the GaInAsP layer 78 in thesemiconductor laser area 70.

[0103] Thus, the layer structure shown in FIG. 12C is formed in thesemiconductor laser area 100, whereas the layer structure shown in FIG.12D is formed in the coupling area.

[0104] Thereafter, a 100-nm-thick p-InP cladding layer having a carrierdensity of 5×10¹⁷ cm⁻³, a 100-nm-thick p-AlInAs layer 82 having acarrier density of 1×10¹⁸ cm⁻³, a 2000-nm-thick p-InP cladding layer 84having a carrier density of 1×10¹⁸ cm⁻³, and a 300-nm-thick p-GaInAscontact layer having a carrier density of 1×10¹⁹ cm⁻³ are grown byepitaxial processes. Thus, the layer structure shown in FIG. 12E isformed in the semiconductor laser area 70, the layer structure shown inFIG. 12F is formed in the optical modulator area 100. FIG. 12G showsthese layer structures in the sectional view taken along the opticalaxis.

[0105] The layer structure formed on the n-InP substrate 72 andincluding p-GaInAsP contact layer 86, p-InP cladding layer 84, p-AlInAslayer 82, p-InP cladding layer 81, p-InP cladding layer 80, GaInAsPlayer 78, MQW 76, and n-InP cladding layer 74 are etched to configure10-μm-wide ridge stripes in both the semiconductor laser area 70 and theoptical modulator area 100, both the ridge stripes being aligned witheach other.

[0106] Subsequently, an oxidation process is conducted to the ridgestripes in the semiconductor laser area 70 and the optical modulatorarea 100 to selectively oxidizing the Al content in the p-AlInAs layer82 from both the sides of the ridge stripes toward the interior thereof,thereby forming a pair of 3.5-μm-wide stripe Al-oxidized regions 88sandwiching therebetween an un-oxidized region of the oxidized layer 82.

[0107] A SiN film 90 is then formed on the semiconductor laser area 70and the optical modulator area 100, followed by selective etchingthereof to form openings therein which expose the underlying p-GaInAsPlayer 86 in both the areas 70 and 100.

[0108] Finally, p-side electrodes 92 and 102 each including Ti/Pt/Aufilms are formed in both the areas 70 and 100 except for a portion ofthe optical waveguide 68, and an n-side electrode including AuGeNi/Aufilms is formed on the bottom surface of the n-InP substrate 72. Thus,the semiconductor optical device 66 is obtained which includes thesemiconductor laser device 70 shown in FIG. 9, the optical modulator 100shown in FIG. 10, and the optical waveguide 68 shown in FIG. 11 andcoupling the semiconductor laser device 70 and the optical modulator100. The semiconductor optical device 66 as obtained in the presentembodiment has excellent device characteristics.

[0109] Since the above embodiments are described only for examples, thepresent invention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

[0110] For example, the AlInAs layer may be disposed on the MQW. TheAlInAs layer may also underlie the MQW, if the p-type cladding layerwithin which the AlInAs layer is disposed underlies the MQW.

What is claimed is:
 1. A quantum confinement Stark effect (QCSE) opticalmodulator comprising a compound semiconductor substrate, and a layerstructure formed thereon, said layer structure including anAlGaInAs-based multiple quantum well (MQW), a pair of cladding layershaving opposite conductivity types and sandwiching therebetween saidMQW, and an Al-containing layer overlying said MQW or formed within oneof said cladding layers having a p-type conductivity, said layerstructure being configured as a ridge stripe at a portion including saidAl-containing layer, said Al-containing layer having a currentconfinement structure wherein a pair of Al-oxidized regions of saidAl-containing layer sandwiches therebetween a central un-oxidized regionof said Al-containing layer.
 2. The QCSE optical modulator as defined inclaim 1 , wherein said Al-containing layer is an AlInAs layer.
 3. TheQCSE optical modulator as defined in claim 1 , further comprising asemiconductor laser device formed on said compound semiconductorsubstrate, said semiconductor laser device having another ridge stripeoptically coupled with said ridge stripe.
 4. The QCSE optical modulatoras defined in claim 3 , wherein said semiconductor laser device is aGaInAsP- or AlGaInAs-based laser device.
 5. The QCSE optical modulatoras defined in claim 3 , wherein at least one layer of said another ridgestripe is coupled with a corresponding layer of said ridge stripe by abutt-joint structure.
 6. The QCSE optical modulator as defined in claim3 , wherein each layer in said another ridge stripe is formed as acommon layer with a corresponding layer in said ridge stripe
 7. A methodfor fabricating a quantum confinement Stark effect (QCSE) opticalmodulator comprising the steps of: forming a layer structure on an InPsubstrate, said layer structure including a multiple quantum well (MQW),pair of cladding layers sandwiching therebetween said MQW and anAl-containing layer overlying said MQW; configuring at least a portionof said layer structure including said Al-containing layer to form aridge stripe; and selectively oxidizing Al in said Al-containing layerto form a current confinement structure in said ridge structure, saidcurrent confinement structure having a pair of Al-oxidized regions ofsaid Al-containing layer sandwiching therebetween an un-oxidized regionof said Al-containing layer.
 8. A method for fabricating a semiconductoroptical device comprising the steps of: forming a first layer structureon first and second areas of an InP substrate, said layer structure insaid first area constituting a semiconductor laser device; forming anarea-selective growth mask on a portion of said first layer structure insaid first area of said InP substrate; selectively removing said firstlayer structure to expose said second area of said InP substrate;forming a second layer structure on said second area, said second layerincluding a multiple quantum well (MQW), pair of cladding layerssandwiching therebetween said MQW and an Al-containing layer overlyingsaid MQW; configuring at least a portion of said second layer structureincluding said Al-containing layer to form a ridge structure;selectively oxidizing Al in said Al-containing layer to form a currentconfinement structure in said ridge stripe, said current confinementstructure having a pair of Al-oxidized regions of said Al-containinglayer sandwiching therebetween an un-oxidized region of saidAl-containing layer; forming a quantum confinement Stark effect opticalmodulator including said ridge stripe on said second area.
 9. A methodfor forming a semiconductor optical device comprising the steps of:forming an area-selective growth mask on an InP substrate; selectivelygrowing a layer structure by using said area-selective growth mask, saidlayer structure including a multiple quantum well (MQW), and a pair ofcladding layers sandwiching therebetween said quantum well on said InPsubstrate; consecutively forming an Al-containing layer and at leastanother cladding layer on said layer structure; configuring said aportion of said layer structure including said Al-containing layer toform a ridge stripe; and selective oxidizing Al in said Al-containinglayer to form a current confinement structure in said Al-containinglayer, said current confinement structure including a pair ofAl-oxidized regions sandwiching therebetween an un-oxidized region; saidselectively growing step except for the step of growing well layers insaid MQW using a material gas including an etchant for removingpolycrystalline substance formed on said area-selective growth mask.